Display system using two-photon fluorescent materials



Nov. 11, 1970 DUGUAY ETAL 3,541,542

DISPLAY SYSTEM USING TWO-PHOTON FLUORESCENT MATERIALS Filed Sept. 15;1967 FIG./

AMPLITUDE MODULATOR FREQUENCY CHANGER BEAM SPL/TTER AMPL/TUDE MODULATORPULSE SECOND D/SPERS/VE HARMON/C GE NE RA TOR POLAR/25R ME D/UM M. A.DUGUAY lNVENTORS J. A GlORDMA/IVE P.M.RENTZE IS ATTORN V Patented Nov.17, 1970 3,541,542 DISPLAY SYSTEM USING TWO-PHOTON FLUORESCENT MATERIALSMichel A. Duguay, Berkeley Heights, Joseph A. Giordmaine, Summit, andPeter M. Rentzepis, Millington, N..I., assignors to Bell TelephoneLaboratories, Incorporated, Murray Hill and Berkeley Heights, N.J., acorporation of New York Filed Sept. 15, 1967, Ser. No. 668,052 Int. Cl.G011 N34 US. Cl. 340324 12 Claims ABSTRACT OF THE DISCLOSURE A displaysystem includes a medium which fiuoresces only upon the absorption oftwo photons, one from each of two coincident signals having differentfrequencies and different intensities. One of the signals is an amplitude modulated train of picosecond image information pulses and theother is a train of picosecond interrogate pulses. Beam deflectors causethe two signals to overlap in appropriate regions of the medium toproduce a sequence of fluorescent spots which constitute an opticalrepresentation of the image information. The display system operates asa detector of picosecond pulses by causing such a pulse to overlapwithin the medium a second pulse of a different frequency.

BACKGROUND OF THE INVENTION This invention relates to display systemsand more particularly to such systems utilizing two-photon fluorescentmaterials capable of displaying picosecond pulses.

Recent developments in the laser art have made it possible to phase-lockthe oscillating modes of a laser by any of several well-known techniquesincluding synchronous modulation and Q-switching. The output of aphaselocked laser is an optical pulse train having a pulse repetitionrate give by 2L, where c is the velocity of light and L is length of theactive medium. More importantly, however, the pulse width of the pulsesgenerated is typically in the picosecond range (i.e., 1( seconds). Suchpulses, which are also produced by stimulated Raman emission, areideally suited to serve as the carrier for an optical pulse codemodulation system. To utilize such narrow pulses in an opticalcommunication system it is necessary to be able to detect the pulses ata receiver. The enormous bandwith required to detect and display suchnarrow pulses, however, is not available in prior art receivers.

Characteristic of the prior art is the luminous spot display devicewhich utilizes a gaseous medium such as mercury isotope 198. Two D.C.light beams generated by separate mercury arc lamps are made to overlapwithin the medium and to cause the medium in the region of overlap toemit green light. The disadvantages of such a system, however, arenumerous. The most fundamental objection is that such mediums arecharacterized by atomic state lifetimes in the order of seconds, muchtoo long to detect picosecond pulses. Furthermore, the display deviceproduces a luminous spot only if the two light beams are non-colinear.If the beams were colinear, the long lifetimes of mediums such asmercury vapor would produce a trace or spot several meters in lengthwhich is, of course, undesirable in any practical displa device. Lastly,the light beams employed are noncoherent D.C. beams which are notreadily modulated to carry optical information and are thereforeunsuitable to serve as carriers in a practical display system.

SUMMARY OF THE INVENTION The present invention employs materials whichfluoresce when a suflicient number of electrons are excited from a lowerto a higher energy state by an excitation signal or combination ofsignals. The electrons subsequently undergo a radiative transition fromthe higher to the lower energy state known as fluorescence. Inaccordance with an illustrative embodiment of the invention a displaysystem utilizes a medium, typically diphenylcyclopentadiene dissolved intetrahydrofuran, which fluoresces only upon the absorption of twophotons per quantum of fluorescent radiation. To produce detectablefluorescence, however, the excitation signal must have a frequency suchthat twice its energy is greater than the energy separation between thelower and higher states. In the case where the excitation signal is acombination of two or more signals at different frequencies, thecondition to be satisfied is that the sum of the frequencies be suchthat the excitation signal energy is greater than said energyseparation. In addition, for the fluorescence to be visible or, ingeneral, detectable the medium must absorb a certain minimum number ofphotons which means that the signal must have a certain minimumintensity.

In accordance with the present invention, one photon per quantum offluorescent radiation is supplied from each of two coincident signalshaving different frequencies and different intensities. The signalsthemselves are typically picosecond pulses generated by a phase-lockedlaser, and the frequency referred to is the optical frequency of eachpulse. Each signal alone does not produce fluorescence because onesignal is of insufiicient frequency to excite electrons across theenergy gap of the medium, Whereas the other signal, although ofsufficient frequency, is of insuflicient intensity to produce detectableflourescence. Where, however, the two signals overlap within the medium,both the frequency and intensity conditions are satisfied andfluorescence occurs in the region of overlap. The medium is particularlysuitable for a two or three dimensional display device because nobackground trace is produced (i.e., a single signal produces nodetectable fluorescence).

The use of a two-photon fluorescent medium, typically a liquid,distinguishes further the present invention from the prior art typifiedby luminous spot display devices utilizing mercury vapor. The atomictransitions which take place in the murcury isotope 198, for example,include a radiative transition from the 7 8 level to the 6 P level. Inthe liquids of the present invention, however, that same transitionwould be radiationless. In contrast two-photon fluorescence involves theexcitation of electrons from a ground level singlet state to a higherenergy level singlet state by the absorption of two photons per quantumof fluorescent radiation, fluorescence occurring when the excitedelectrons fall back to the vibrational levels of the ground state Acomplete display system in accordance with the invention employs asource of picosecond pulses and appropriate modulators to impress imageinformation onto one pulse train and logical or interrogate informationonto another. Beam deflectors cause the two pulse trains to overlap atappropriate positions in the medium to produce a sequence of fluorescentspots which constitute an optical representation of the imageinformation. A unique feature of the invention is that the two pulsetrains (which may be optical picosecond pulses generated by aphaselocked laser) may be directed into the medium along colinear pathsand still produce fluorescent paths and still produce fluorescent spotsonly in the regions of medium where an image pulse and an interrogatepulse overlap.

In addition, in many cases it is desirable to measure the pulse widthand pulse repetition rate of a picosecond pulse train. The prior art hasresorted to certain indirect methods of measurement includingcoincidence techniques which utilize nonlinear (e.g., electrooptic)crystals that generate as an output the sum and difference frequenciesof two coincident signal inputs. To detect a pulse from a phase-lockedlaser, for example, the pulse is split into two signals and passedsimultaneously through the crystal. The output of the crystal isdetected. By inserting a variable time-delay into the path of one of thesignals, the output can be reduced to zero. The amount of delay insertedis then an indirect measure of the pulse Width. However, themeasurements cannot be accurately made from a single pulse, rather manypulses are required to properly adjust the delay and reduce the outputto zero.

A picosecond pulse detector in accordance with the present inventionutilizes two-photon fluorescent materials of the type previouslydescribed. To detect a picosecond pulse of frequency :0 for example, thepulse is passed through a polarizer and a second harmonic generator toproduce a pulse at frequency 2 Both pulses are then passed through adispersive medium, the result being that the pulse at 211: is delayed.Finally, both pulses are directed colinearly into a two-photonfluorescent medium. A mirror disposed at one end of the medium andnormal to the pulse path transmits the pulse at 2w but reflects thepulse at w Consequently, the pulse at te after having been reflected,overlaps the delayed pulse at 2w The intensity I of the pulse at m istypically one hundred times greater than the intensity I of the pulse at200 and consequently fluorescence is produced only in the region ofoverlap within the medium. The fluorescent spot produced is aconvolution of the overlapping pulses, the intensity of the spot beingabout aI I At, where a is a constant and At is the pulse width, and thelength of the spot in the medium being proportional to the pulse width.

BRIEF DESCRIPTION OF THE DRAWINGS The invention, together with itsvarious features and advantages, can be easily understood from thefollowing more detailed description taken in conjunction with theaccompanying drawings, in which:

FIG. 1 shows schematically the energy level diagram of a two-photonfluorescent material utilized in the present invention;

FIG. 2 shows schematically a display system in accordance with oneembodiment of the invention; and

FIG. 3 shows schematically a picosecond pulse detector in accordancewith one embodiment of the invention.

DETAILED DESCRIPTION In general, detectable fluorescence involves twoparameters, frequency and intensity. Frequency is related (by Plancksconstant) to the signal energy required to excite electrons across theenergy gap of the medium employed. Intensity, on the other hand, isrelated to the total numbers of photons supplied by an incident signal.Signal intensity is therefore related to the total number of photonsabsorbed by the medium which is in turn a measure of the fluorescentintensity produced. Thus, a signal may fail to produce detectablefluoresence either because its frequency is too low to excite electronsacross the gap, or, although it may have the proper frequency, becauseits intensity is too low to cause the medium to absorb a suflicientnumber of photons. The latter properties are exploited in the displaysystem of the present invention to eliminate background trace and toproduce a fluorescent spot only in the region of the medium where twocoincident signals overlap, as will be more fully described below.

The present invention employs a medium which requires the absorption oftwo photons to produce each quantum of fluorescent radiation.Furthermore, by appropriate choice of the frequencies and intensities ofa pair of optical signals, detectable fluorescent radiation is producedonly in the region of the medium where the signals are made to overlap,one photon being absorbed from each of the two signals per quantum offluorescent radiation. Turning now to FIG. 1 there is shownschematically an energy level diagram for a two-photon fluorescentmedium characterized by a pair of singlet states S and S separated by anenergy gap E Associated with each singlet state are a plurality ofvibrational levels V and V respectively. Generally, fluorescence takesplace upon the absorption of two photons per quantum of fluorescentradiation which causes electrons to be excited from S to S or to V Inthe latter case, the electrons subsequently undergo a nonradiativetransition from V to S from which state electrons fall to V accompaniedby fluorescence F. Two signals are employed to produce two-photonfluorescence, one of frequency f and intensity I and the other offrequency f and intensity I The signal at f is made to be such that theabsorption of two photons from it alone does not excite electrons acrossthe energy gap; that is, 2hf E On the other hand, the signal at f ismade to be such that the absorption of two photons from it aloneproduces fluorescence (i.e., 2hfz E but not detectable fluorescence.This is accomplished by maintaining its intensity low, typically I -I100, which means that the total number of quanta of fluorescentradiation produced by the signal at f is not high enough to be detectedby the human eye (in the case of a visual display). Thus, neither signalalone produces detectable fluorescence and consequently neither producesa background trace. Where, however, the signals overlap within themedium both the conditions of frequency and intensity for detectablefluorescence are met. That is, the energy of the combined signals issufficient to excite ele trons across the gap since h(f +f E and, theintensity of the combined signals produces a total number of quantawhich is detectable. In summary, then, where the two signals arecoincident and overlap, a fluorescent spot is produced in the region ofoverlap. But where the signals do not overlap, or are not coincident, nofluorescence and therefore no background trace is produced.

These principles of two-photon fluorescence are embodied in a displaysystem depicted schematically in FIG. 2. A cell 12 contains a two-photonfluorescent medium, typically pyrene, diphenylcyclopentadiene ortetraphenylcyclopentadiene dissolved in a solvent such astetrahydrofuran. For the purposes of discussion the cell 12 has beendivided into sixty-four imaginary subcells, eight on a side. Thesubcells represent regions of the medium in which two signals will bemade to overlap and produce a fluorescent spot in accordance with imageinformation to be displayed. A pulse source 14, typically a phaselockedlaser, generates a train of picosecond pulses at an optical frequency fsuch that 2hf E The optical frequency is to be distinguished from thepulse repetition rate which is c/ 2L, where c is the speed of light andL the effective length of the laser cavity. The pulse train is thendivided into two separate signals or pulse trains at the same frequencyf by the beam splitter 15. One of the pulse trains is passed throughamplitude modulator 16 where it is encoded with image information to bedisplayed. In the example of the 8 x 8 subcell system, this pulse traincontains originally eight pulses each separated by a length in space of2d, where d is the width of each subcell. The third and seventh pulsesare deleted by the modulator 16 for the purposes of illustration. Theother pulse train is passed through a frequency changer 18 (e.g., a KDPcrystal) which produces at its output a pulse train at an opticalfrequency f which is typically the second harmonic of f (i.e., f 2f Theintensity of these pulses is however typically about one hundred timesless than the intensity of the information pulses. The pulse train at 2is then encoded by amplitude modulator 20 to produce a train ofinterrogate pulses separated in space by a distance 8d. Beam .5deflector 22 positions the information pulse train along a pathcorresponding to row 1 of cell 12. Beam deflector 24 similarly positionsthe interrogate pulse train along a path colinear with the path of theinformation pulse train. The pulse spacing of the respective trains issuch that the first interrogate pulse overlaps each information pulse inthe appropriate subcell to produce a fluorescent spot in accordance withthe image information. (In general, if each row of cell 12 is dividedinto n imaginary subcells of width d, then information pulse train andthe interrogate pulse train have respective pulse spacings of 2d andmi.) The intensity of the spot can be controlled of course by varyingthe amplitude of the optical pulses. More specifically, interrogatepulse 1 overlaps information pulse 8 in subcell 8. As the pulsespropagate colinearly along row 1, the interrogate pulse 1 overlaps inturn information pulse 7 in subcell 7, and so forth. After the firsteight information pulses (i.e., the first frame) are interrogated, asecond frame of information pulses is positioned along row 2 by beamdeflector 22, and interrogate pulse 2 is similarly positioned along row2 by beam deflector 24. The process of overlapping pulses and producingfluorescent spots repeats as before. In this manner, a pattern offluorescent spots is produced in the cell 12, the spots producing anoptical representation of the image information.

Optical modulators and beam deflectors are both well known in the artand include, for example, those of the electrooptic type.

Although the cell 12 is depicted as being two dimensional, it is ofcourse readily possible to devise a three dimensional display by merelyplacing the fluorescent medium in cubic-like volume and causing theinformation and interrogate pulses to overlap within appropriate regionsof the three dimensional space. Similarly, a three dimensional colordisplay can be constructed, for example, by positioning a plurality ofcells on top of one another, each of the cells containing a diflerenttwo-photon fluorescent medium each capable of "emitting a differentcolor light. Each cell could then be interrogated separately to producethe proper blend of each color in each fluorescent spot. Alternatively,a color display could be devised by multiplexing several frequencies onthe same pulse train in combination with a suitable two-photonfluorescent medium.

In addition to a display system, the present invention contemplates theuse of a two-photon fluorescent medium as a picosecond pulse detector asshown in FIG. 3. A picosecond pulse at optical frequency f (generated byphase-locked laser 30, for example) is polarized by polarizer 32 andthen passed through a second harmonic generator 34, typically a KDPcrystal. The output of the generator 34 consists of a pulse at frequencyf and one at the second harmonic 2 1, the intensity of the secondharomnic being typically one hundred times less than that offundamental. These pulses actually travel along colinear paths, but forthe purposes of clarity the paths have been separated in FIG. 3 into twoparallel paths. The two pulses are first directed into a dispersivemedium 36, typically bromobenzene, which delays the second harmonic withrespect to the fundamental, and are then directed into one end oftwo-photon fluorescent medium 38. At the other end is disposed a mirror40 which is transparent to the second harmonic but not to thefundamental. The fundamental strikes the mirror first, is reflected, andsubsequently overlaps the second harmonic producing a fluorescent spotwithin the region of overlap. The shape of the spot, whose intensity andlength are proportional respectively to the pulse intensity and width,is a convolution of the shapes of the overlapping pulses and may bephotographed by a camera system comprising a filter 42 which absorbs thelight at frequencies and 2h but transmits the fluorescent light, a lens44 and a photographic plate 46. The advantage of elimination of thebackground trace here is the ability to display weak picosecond signalswhich could otherwise be obscured in the background fluorescence.

In a specific example, the medium 38 comprises a 5 x10" M solution ofdiphenylcyclopentadiene (DPCPD) in tetrahydrofuran which exhibits afluorescent maximum at 043 The laser is typically a phase-locked Nd:glass laser which generates a train of picosecond plane-polarizedpulses spaced 3.3 nsec. apart at an optical wavelength A =c/f =l.06 Thesecond harmonic generator 34, a KDP crystal set at the phase-matchingangle, produces at its output a train of pulses at an optical wavelengthx :c/f =0.53 in addition to the train of pulses at 1.06 The intensity ofthe 1.06 1. pulse train is about 1 G watt/cm. and that of the 0.53 pulsetrain about 1 M watt/cm. The composite beam is then passed through adispersive medium 36 of bromobenzene having a length chosen so thatdispersion causes the 1.06 pulses to emerge from the dispersive medium30 psec. ahead of the 0.53 pulses. The beam finally enters a 2 cm. longcell containing a DPCPD solution and strikes at normal incidence thedielectric mirror which typically has about 75% reflectivity at 1.06 andless than 5% reflectivity at I 053 1.. The fluorescent patterns producedin the DPCPD cell 38 may be recorded at 3000-speed Polaroid film 46 bymeans of an ;f/4 camera 44 viewing the cell through a dark blue Corning7-59 filter 42. The filter allows the DPCPD fluorescence (0.46 to 0.40to be recorded, but absorbs the 0.53 and 1.06,.t scattered light.

What is claimed is: 1. Optical apparatus comprising a display devicecomprising a medium having an energy gap defined by a higher and a lowerenergy state, the separation of the energy states being such as torequire the absorption of two photons to excite electrons from the lowerto the higher energy state, and being characterized by a transition fromthe higher to the lower state which produces fluorescence, means forgenerating a first signal having a frequency such that simultaneousabsorption by the medium of two photons from the signal is suflicient toexcite electrons from the lower to the higher energy state, and being ofan intensity insuflicient to produce detectable fluorescence, means forgenerating a second signal having a frequency such that the simultaneousabsorption of two photons from the signal is insuflicient to exciteelectrons from the lower to the higher energy state, and being of highenough frequency and intensity to produce detectable fluorescence whenthe first and second signals simultaneously overlap within a region ofsaid medium, and means for producing within said medium an opticalrepresentation of the image information carried by one of said signalsby causing said signals to overlap within a region of said medium and toabsorb simultaneously a single photon from each of the signals perquantum of fluorescent radiation, thereby to produce a fluorescent spotin the region of overlap. 2. The optical apparatus of claim 1 whereinsaid second signal generating means comprises a source of pulses ofoptical energy, and said first signal generating means comprises asecond harmonic generator, the second signal being directed into saidsecond harmonic generator.

3. The optical apparatus of claim 1 wherein said means for causing thesignals to overlap comprises means for delaying one of the signals withrespect to the other comprising a dispersive medium, means for directingthe delayed signal and the other signal into one end of said mediumalong colinear paths, a reflector disposed at the other end of saidmedium normal to the path of the signals, said mirror being highlytransparent to the delayed signal and highly reflective to the othersignal.

4. The optical apparatus of claim 1 wherein said medium comprisespyrene.

5. The optical apparatus of claim 1 wherein said medium comprisestetraphenylcyclopentadiene.

6. The optical apparatus of claim 1 wherein said medium comprisesdiphenylcyclopentadiene.

7. The optical apparatus of claim 6 wherein the first signal has awavelength of about 0.53;; and the second signal has a wavelength ofabout 1.06

8. The optical apparatus of claim 6 wherein the first signal has anintensity of about one megawatt per square centimeter and the secondsignal has an intensity of about one gigawatt per square centimeter.

9. The optical apparatus of claim 1 in combination with means formodulating said one of said signals to carry image information,

means for modulating the other signal to produce interrogate pulses, and

wherein said means for producing within said medium an opticalrepresentation of one of the signals comprises means for producing anoptical representation of the image information carried by said onesignal.

10. The optical apparatus of claim 9 wherein said signals are directedalong colinear paths from opposite directions and said medium is dividedin n imaginary cells of width d along the colinear path, and wherein theimage information signal comprises a pulse train having a pulse spacingin space of 2d and the other signal comprises a pulse train having apulse spacing in space of mi.

11. The optical apparatus of claim 9 wherein said second signalgenerating means comprises a phase-locked laser, and said first signalgenerating means comprises a frequency changer, and means for directingthe first signal into said frequency changer.

12. The optical apparatus of claim 11 wherein said frequency changercomprises a second harmonic generator.

References Cited UNITED STATES PATENTS 3,123,711 3/1964 Fajans 2507l3,253,497 5/1966 Dreyer 2507l 3,296,594 1/1967 Van Heerden 331-945THOMAS B. HABECKER, Primary Examiner M. M. CURTIS, Assistant ExaminerUS. Cl. X.R. 250-71; 331-945

